答案在文档中bindParam：

与PDOStatement :: bindValue（）不同，该变量被绑定为引用，并且仅在调用PDOStatement ::
execute（）时进行评估。

和 execute

调用PDOStatement :: bindParam（）将PHP变量绑定到参数标记：绑定变量将其值作为输入传递，并接收其关联参数标记的输出值（如果有）

例：

$value = 'foo';
$s = $dbh->prepare('SELECT name FROM bar WHERE baz = :baz');
$s->bindParam(':baz', $value); // use bindParam to bind the variable
$value = 'foobarbaz';
$s->execute(); // executed with WHERE baz = 'foobarbaz'

要么

$value = 'foo';
$s = $dbh->prepare('SELECT name FROM bar WHERE baz = :baz');
$s->bindValue(':baz', $value); // use bindValue to bind the variable's value
$value = 'foobarbaz';
$s->execute(); // executed with WHERE baz = 'foo'